February 2000 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - February 2000 Issue

The Telescope Nut
by Jeff Baldwin

Autocollimating

There is a very simple, math-free method of testing telescope mirrors, and it is extremely precise. I'm referring to autocollimation. It tests not only your mirror, but rather the entire optical system, both primary and secondary mirrors. Here is an illustration of the set-up.

The entire telescope is benched up so that it is looking at an optical flat, shown on the left. This flat is a precision mirror that is extremely smooth. It doesn't have to be perfectly flat--it may be slightly spherical, either convex or concave--but it must be smooth. A light source is set up at the focus of the telescope. The light passes a razor, projects to the secondary mirror, reflects toward the primary mirror, then on to the test flat. Once it reflects off the test flat, the light retraces its path back to the razor where it passes and creates a slit pass and is viewed by the optician.

It looks just like a Foucault knife-edge test when viewed, except it has more contrast. Notice that the light reflects off each of the two mirrors in the telescope twice. If there are errors on these mirrors, their illusion of error is doubled as a result of this double pass. This increases the contrast of the error, and allows errors of half the normal size to be detected, making the test better than 1/20 wave. When the system is perfectly figured, the illusion is null rather than doughnut shadowed. Null tests require no math. If the shadows are on the same side as the razor, the zone is under-corrected, and if the shadows are on the opposite side, the zone is over-corrected.

The test mirror is aluminized, and the secondary mirror is also most likely aluminized. The primary mirror may not be aluminized since it may still be under corrective work and being the element tested for correction. This means that instead of reflecting 4% of the light as in the Foucault knife-edge examination, it is going to reflect 4% of 4% of the light, since it reflects off an unaluminized surface twice. This makes 0.16% reflection, times 88% twice for the secondary mirror and 88% for the test flat, all multiplying to 0.11%. That's a lot of light to be missing, so a brilliant light source is needed. Super bright LEDs may not be bright enough for this, and mag lights are too hot to have near the face. This is why many autocollimation testers use beam splitters. This allows the light to come from one place and look like it's coming from you. This also knocks off another 55% of the light, leaving only 45% of 0.11%, or about 0.05% of the light.

For larger mirrors, autocollimation is unaffordable due to the expense of large optical flats. In this case some opticians have resorted to using a puddle of oil as their flat. They suspend the telescope in test above the oil puddle, which resumes the shape of the Earth's surface, which is spherical. If it is not too large it will be very close to flat. Keep in mind that autocollimation mirrors don't have to be perfectly flat, but must be smooth, so the puddle system actually works. Vibrations from cars, trucks and footsteps can destroy the image.

Our club has a 12" optical flat made by member Frank Kocsis. I have a stand with adjustable motions, and will have a new razor-light source eyepiece again soon. This will allow members to test optics up to 10 or 11 inches in aperture with extreme accuracy. It is difficult to set it up perfectly, and due to the dimness of the test, it must be done in a very dark environment.

Even though the information is astounding, after any optical bench test is performed, the telescope should not be considered finished until a star test is performed.

Clear Skies...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.